Friday, March 21, 2014

vogl's UI (being worked on by Peter Lohrmann) has come far in the past month. I used it today while debugging what seemed to be a replay bug in Xonotic (reported by a dev named blackout24 on github). I first trimmed a single frame that clearly showed the problem, then played back this trimmed trace in a endless loop to verify the issue still showed up in the trim. I manually trimmed the source trace using voglreplay64, but I think initial support for doing all this directly from the UI just went in.

The UI helped me quickly pinpoint the first draw affected by the rendering problem. I then drilled down and examined all the GL state, textures, shaders, etc. on and around this draw. Clicking on a GL command that already had a snapshot was fast, only around a second in debug, and around 3-4 seconds on commands without snapshots. I still dumped the trimmed trace to JSON+loose files, more out of habit than anything, but using the UI was much faster than doing things the old way (which involved dumping massive amounts of PNG's on each major event, then using voglreplay -find and/or grep on huge JSON files).

Here's the pinpointed draw showing the problem (a completely opaque foliage billboard that should have been rendered transparently):

Depth and stencil buffers are currently displayed by mapping their individual bytes directly to image components - we're working on that.

Here's the foliage texture. I enabled alpha blending in the UI to double check the texture's alpha channel was reasonable:

Xonotic replay showing the problem, with the powerful QtCreator IDE in the background:

Turns out the problem was caused by Xonotic's usage of alpha to coverage on a multisampled default framebuffer. We don't currently support automatically enabling multisampling on default framebuffers during replaying. (We do of course support MSAA renderbuffers/textures/FBO's, but not on the default framebuffer yet.)

For now, I added a "-msaa X" command line option to the replayer to enable MSAA on the default framebuffer until we address this. This is crappy, but the vast majority of GL apps just don't enable MSAA this way and we have bigger fish to fry at the moment. (Also, I don't want to touch vogl's GLX/X-Windows related code until we abstract it away into SDL or something.)

Wednesday, March 19, 2014

Steam's Big Picture mode is one of the last remaining Valve OpenGL apps that vogl didn't support until now. (The desktop client's GL callstream has worked for months.) The fixes for Big Picture are now all pushed to our github repo.

Here's a Big Picture ("10ft") replay in interactive mode after pausing (which involves a full state snapshot, context teardown, and state restore) and continuing playback:

Replaying 10ft traces on NVidia technically works, but there's a driver bug that is causing playback to be extremely slow on my box (that NVidia is checking out). So all tracing and replaying in these shots was done on a AMD 57xx series part using the closed source fglrx driver.

The desktop ("2ft") GL callstream is looking good too, but compared to 10ft I have hardly spent any time looking at it. (I used the "-lock_window_dimensions -width 2560 -height 1600" cmd line options to replay this trace for 10ft, so the window is much bigger than needed for 2ft):

There are some known remaining issues, none of them show stoppers for debugging purposes. I'll be adding this trace to our shiny new regression test system Mike Sartain is working on soon.

- The replayer's auto window resize logic is almost useless on Steam traces because it creates so many trampoline contexts (associated with tiny windows) during startup and mode changes. So you must currently replay using "-lock_window_dimensions -width X -height Y".

- Can't make single/multi-frame snapshots of 10ft during tracing, only replaying. This isn't a big deal, because you can make a full-stream trace and just trim the frames you want to look at.

This problem is caused by the 10ft renderer keeping several buffers mapped all the time. I have a safe and easy fix coming that might address this issue (but it'll only work when the app keeps the entire buffer mapped).

- Can only debug 10ft on AMD until the NVidia driver bug is fixed

- The UI has not been tested on 10ft traces yet. Peter Lohrmann just added better support for debugging traces containing multiple contexts (specifically to help 10ft debugging along) which I'll try soon.

- The 10ft renderer deletes textures while they are still bound to FBO's (and keeps the FBO's around)

This causes various problems for the snapshot code because it can't retrieve the texture attachment handles in these FBO's (we just get 0's for the GL_FRAMEBUFFER_ATTACHMENT_OBJECT_NAME), and (the last time I checked) we can't reliably retrieve information on these deleted textures on all drivers. This seems to be a very rare pattern (that I've never seen in any game titles, just 10ft). After asking around it turns out this problem is not a showstopper for 10ft because it always rebinds a new texture to the same attachment point before it ever renders to the FBO again.

All that red text in the below screenshot is due to this issue, but the output should still be correct.

You don't need to do anything special to trace steam:

./trace.sh /usr/bin/steam

trace.sh is our example tracing script, see here. The example script causes the tracer to wait for a keypress, but you may not see the "waiting for keypress" message - just press any key if the app appears to stop.

Saturday, March 15, 2014

We've been using Mercurial+TortoiseHg for the previous year (with hosting on Bitbucket), but the open source mainstream uses git so we're now switching vogl over to it exclusively. I gather most Linux devs primarily use command line tools, which is fine and all (I obviously do too when needed) but I want to find good GUI's for this stuff. The last time I had to use CLI tools for version control was 1997 under DOS.

There's an added bonus to being obstinate and pushing to find and use good native Linux GUI's for our major devtools: devs porting from the Windows/OSX/console worlds already have huge piles of solid GUI-based tools, and we need to find reasonably competitive native Linux alternatives. (When I say "native", I mean "not under Wine". I use Wine every day to run some old non-critical Windows programs I just can't find Linux alternatives for that I like, such as the Boxer Text Editor and Paint Shop Pro. Wine seems to run older Win32 apps better than Windows 7/8 itself these days!)

So I'm on the lookout for a git UI that is at least as good as TortoiseHg for doing the basics. I found a good Visual Studio alternative (QtCreator) a year ago after a wide search involving around a half dozen other Linux IDE's. I knew QtCreator was a good product after using its debugger for 20 minutes. It's by no means perfect but I've not had to use gdb/cgdb once since switching to it.

SmartGitHg has been on my radar, so I'm trying it out. It's commercial but has a 30 day trial (and is free for non-commercial use). This thing could be unusable -- I have no idea yet.

It needs openjdk-7-jre to run, which I installed first using the Muon package manager (under Ubuntu 13.10+KDE). The UI seems more complex, but cleaner, than TortoiseHg's. If you're already familiar with Mercurial/thg it seems pretty easy to map over the concepts and accomplish the basics. I just pushed a trivial change up using it (added a link to the vogl wiki). I'll keep trying UI's if necessary until something works for 90% of the things devs do (add files, check in, push, pull, merge, resolve conflicts, browse history, etc).

Friday, March 14, 2014

I fixed a number of issues specific to AMD's driver - changelist notes are here. Mike should hopefully push these changes to github out tonight or tomorrow latest. (3/15: These changes are live on github - thanks Mike!)

Here's Dota2 replaying on fglrx in -interactive mode. Also, our regression test suite is now working (for the first time!) on AMD, which is pretty exciting.

The GL API callstream involved is hairy - it's kinda amazing that it works at all:

- I traced Dota2 using apitrace on an NVidia 780 to a .trace file
- I played this back on AMD's fglrx using glretrace, then intercepted its output using libvogltrace to a vogl .bin trace file
- I then play this new trace using voglreplay. The regression test suite verifies that the backbuffer CRC's seen during tracing vs. replaying are the same (we've failed if not).

So we're mixing two different drivers and tracing/replaying frameworks in this test. The yellow warnings in the screenshot below are caused by missing uniforms, which are optimized differently by the AMD driver's compiler vs. NVidia (so some program uniforms are missing on AMD, which should be harmless).

Wednesday, March 12, 2014

All debuggers have limitations. Most of the time, you don't really know what they are until they pop up while you're trying to debug something (usually at the worst time, after wasting many hours). So here's a list of vogl limitations/issues I've been compiling (which will go up on the wiki once it's setup):

I would like to support it, but honestly it's challenging enough to do this on vanilla desktop stacks. Once you throw 2 drivers in there all bets are off with all the tracers I've tried. Any help in this area would be great.

We do support manually loading our tracer (libvogltrace32/64.so) on Optimus, but it's not something I've had the time to test much. To do this, manually load libvogltrace and dlsym() the gliGetProcAddressRAD()function (to be renamed to voglGetProcAddress()).

Can't take state snapshots during tracing or replaying while any buffers are currently mapped.

This is typically not a problem because almost all apps just map a buffer, poke around inside the mapped region (reading and/or writing with the CPU), then unmap and move on.

I'm currently working on removing this restriction during replaying (which is easy because we fully control all GL contexts during replaying), but reliably removing this limitation during tracing in all scenarios seems challenging.

PBO (pixel pack and unpack buffers) not supported in the current github drop

This is already implemented and is being tested with Steam 10ft traces. I'll hopefully push it up by the end of the week.

GL 4.x is not supported for full-stream or snapshotting

There's a lot of GL 4.x stuff that will work, but it's not been a priority to support the latest bleeding edge stuff. Almost all shipped GL products I'm seeing only use GL 3.x, at best. Interestingly, the biggest/most ported releases tend to use a very conservative set of GL v2/v3.

Cubemap arrays not supported for snapshotting yet (but are OK for full-stream)

Here's the list of texture types we can snapshot: 1D, 2D, RECTANGLE, CUBE_MAP, 1D_ARRAY, 2D_ARRAY, 3D, 2D_MULTISAMPLE, and 2D_MULTISAMPLE_ARRAY. Incomplete textures are OK, but you'll get a warning if you haven't properly set GL_TEXTURE_MAX_LEVEL (which you most definitely should always do because not doing so is unreliable in practice).

Abuse of GL handles+multiple contexts

Sadly GL handles behave in interesting and obscure ways once you introduce sharelists. So before you delete textures (and most other objects) you should make sure they are not bound on other contexts before you delete them, otherwise you're going down a direction that you'll probably regret (and that will give vogl headaches). vogl will give you errors on this scenario when you try to snapshot. For example:

Let's say you create a second context that shares with your first context. It gens a texture (handle=1), binds it on both contexts, calls glTexStorage() to initialize it, then deletes the texture on the 1st context. Everything appears as expected on the 1st context: the texture becomes auto-unbound, glIsTexture() reports false, and I can't retrieve the texture's width anymore (using glGetTexLevelParameteriv()). All nice and neat.

But on the 2nd context, the texture remains bound, glIsTexture() returns false, but I can still retrieve the texture's width. If I call glGenTextures() handle 1 gets immediately reused, even though it's still bound (as reported by glGet() on GL_TEXTURE_BINDING_2D) and even though I can retrieve texture 1's width. At this point handle 1 means two different things (!) on this specific context, which is most wonderful. If I then rebind texture handle 1 (which was just re-genned) I can no longer retrieve the width.

Can't snapshot textures after they are deleted (but still bound elsewhere)

We support snapshotting shaders that have been attached to programs and then immediately deleted. We also support snapshotting programs that have been deleted but are still bound. These are pretty common GL patterns we've seen in a few major titles. At program link time we make a deep copy of all attached shaders (called the "link time snapshot" in the code), so we can guarantee we can snapshot and recreate the program's actual linked state no matter what the app does with the shaders after linking.

However, there are other scenarios (such as binding a texture to a FBO, then deleting the texture but keeping it bound to the FBO) that we don't fully support for snapshotting. This scenario may never be fully supported: the last time I tried I couldn't query state of deleted (but still bound) textures on at least one driver, and we're not going to deeply shadow all texture state to work around this. Luckily, I've only ever seen this done purposely in one app so far, and the attached texture was not actually used for rendering purposes after the deletion. (They kept it attached to keep their hands on the GPU memory so the driver wouldn't reclaim it.)

vogl will spit out an error and typically try to continue snapshotting when it encounters a handle attached to an object that has been deleted (and we've lost track of). You'll get a handle remap error, because we won't know how to remap the handle from the GL replay domain back into the trace domain. The snapshot may cause the replayer to diverge, though.

During replaying the default (GLX) framebuffer is always 32-bit RGBA, no MSAA, with a 24/8 depth stencil buffer.

On the todo list, but this hasn't been a problem so far. Apps that use MSAA tend to use renderbuffers or maybe MSAA textures, probably because this is more portable (vs. mucking around with the default GLX framebuffer's setup). It's possible for an app replay to diverge if the default framebuffer has a configuration that it didn't have during tracing, but in practice I haven't seen this happen.

Replay window auto-resizing can be a problem in some apps

Unlike apitrace, we only use a single replay window and resize it as needed. The auto-resize logic can get stuck resizing too much. This problem pops up most often in GLUT/FreeGLUT apps. We can capture/replay them, but the replayer's window code tends to get confused by the GLUT UI window activity. It'll still replay properly, but slowly as the replayer auto-resizes the replay window.

If the window auto-resizes too much use "-lock_window_dimensions -width X -height Y" on the voglreplay command line to lock the replay window to a fixed size.

We may switch to apitrace-style multiple windows, or maybe pbuffers, to work around this (needs investigation).

We can't snapshot inside of glBegin/glEnd regions.

We didn't think it was worth the extra complexity to be able to snapshot/restore incomplete glBegin sequence, so either snapshot right before or right after the region. (Hey, at least we support snapshotting apps that use glBegin at all!)

Display list limitations

No recursion and no resources can be bound in the display list but textures. We do support around 400 API's inside of display lists. GL display lists are ancient API's at this point, so I don't think we'll do much more in this area unless a big title from the past uses them. (We do already support Doom3's usage of GL display lists, though.)

Be careful deleting contexts that share lists with other contexts

We support tracing/replaying/snapshotting/restoring the state of multiple contexts. vogl has the concept of "root" contexts and "sharelist groups". A sharelist group is 2 or more contexts that share objects, and the first context created in this group (that doesn't, and can't, share with anything else) is marked as the "root" context for that group.

vogl can't snapshot state if the "root" context of a sharelist group is destroyed while other leaf contexts are still present. Either snapshot immediately after all the leaf contexts are destroyed, or reorder your context deletions so the root gets killed last. In 99% of cases none of this matters; most apps just delete all their contexts at once or just leak them at exit.

Forking while tracing

I've encountered problems with this on some apps (mostly Mono ones I think). Needs investigation, we haven't tested it.

Try to delete your contexts when exiting

We've got several hooks in there to make sure the trace is properly flushed and closed when apps exit and leak their contexts. These hooks work most of the time, but it's best if you properly tear down your contexts when you exit.

The replayer does support unflushed traces (with no trace archive at the end), but there are no guarantees.

Also, not properly tearing down your contexts before exiting actually makes it very difficult for us to fully flush any in-progress asynchronous PBO readbacks (used for real-time JPEG capturing).

UI limitations

The entire UI is still very, very new. The texture, renderbuffer, and default framebuffer viewer in particular is very basic. It has little support for viewing traces that have multiple contexts.

Peter Lohrmann is working on improving the UI. We're currently using it to help us debug the debugger itself, which is progress, but there's a bunch of work left before I would try using it to debug a title with it.

Driver compat

I've tested the most on NVidia, a moderate amount on AMD, and (unfortunately) very little on Intel's open source driver so far. (Not purposely - it's just a time limitation.) We mostly ping-pong between NVidia and AMD as driver bugs pop up and we wait for the vendor to provide us with fixes. A developer at LunarG is now helping us get vogl working on Intel's open source driver.

Program binary gotchas

If you trace a 32-bit app that uses program binaries, on at least 1 driver (NVidia) you must replay using the 32-bit replayer (same for 64-bit). You can forcefully disable the app's usage of program binaries while tracing using --vogl_disable_gl_program_binary. This flag causes the tracer to remove the GL_ARB_get_program_binary extension string, and it'll also force the driver to always fail links with program bins (in case you don't check the string).

We've gone back and forth with always disabling program binaries by default in the tracer, but at the end of the day we take the policy of changing the app's behavior during tracing as little as possible unless you have purposely chosen to override something.

Note program binaries are usually *extremely* fragile, so traces containing program binaries may only be replayable on the exact driver version you captured them on.

Can't take a snapshotting while tracing if other threads have contexts current

We take the snapshot immediately after the next glXSwapBuffers() call. The tracer will attempt to make each context current on the same thread that calls glXSwapBuffer()'s so it can take a snapshot, but it won't be able to do this if the app has the context current on the other thread. So don't leave your contexts current across swaps if you want to take a snapshot. (We couldn't think of a reliable/robust way around this limitation.)

To snapshot during tracing, write a file named "__trigger_capture__" to the app's current directory and the tracer will immediately take a snapshot. You can take as many snapshots as you want while tracing. (Of course, you can't have specified "--vogl_tracefile X" on your command line, which would have put the tracer into full-stream mode.) I'll better document this within a day or so, for now just search the code in vogl_intercept.cpp.

You can still try to replay this trace, but it may diverge or horribly fail. To see a more detailed whitelist, run the "voglgen" tool with the -debug option in the glspec directory.

Some of the newer GL debug related funcs aren't in the whitelist yet, I'll be adding them in very soon.

You'll get warnings if you call GetProcAddress() on GL/GLX functions that are not in the whitelist. This is typically harmless, most apps use GL extension libraries that retrieve the addresses of hundreds to thousands of GL funcs they never actually call.

Anyhow, the version of miniz on Google Code only supports zip32, but I added full support for zip64 and a bunch of other features in my spare time last year. I used vogl to test the new code, which you can find the source to here:

The files are marked ".cpp" but it's just plain C code. I need to re-run the latest new code through a C compiler again, but there shouldn't be anything in there that C can't handle. If there is I'll fix it. zip64 was a real pain to fully implement, and next time I will definitely choose a cleaner archive format.

I need to extract this code from the vogl codebase (should be relatively easy as miniz is an independent blob of code) and do a standalone release at some point.

miniz is probably one of my most popular open source libraries. Between all the Microsoft games that used my earlier lossless codecs (Age of Empires 1/2, Halo 3 and I think one of its sequels, Forza 2, Halo Wars) and miniz my compression code has found its way into a bunch of shipped products. One of my other compression libs (picojpeg) is now in orbit on the Skycube nano-satellite, which should be fully deployed from the ISS by the end of the month after its shakedown period is over. I do compression stuff purely for the fun of it so it's pretty cool to see what people wind up doing with it.

Creating a OpenGL debugger that handles both full-stream tracing *and* state snapshotting (with compat profile support to boot!) is a surprisingly massive undertaking for ~3 devs, so please bear with us. We're knee deep in fleshing out the UI and improving the tracer/replayer to be fully compatible with GL v3.3 (4.x will be later this year). Please file bug reports on github and send us trace logs (or apitrace/vogl traces), etc. and we'll do our best to make it work with your app.

We'll be posting more instructions and our current TODO list on the wiki soon.

We're currently in the process of adding PBO support (done, testing it right now), and we've added the ability to snapshot while buffers are mapped during replaying. (Both things are needed to trace/replay/snapshot Steam 10ft.)

Tuesday, March 11, 2014

This is old news by now; I think the press picked up on this even before I heard it was finally released. I really wish we had the time to package it better (so you could actually compile it!) with some examples, etc. There's a ton of practical Linux GL driver know-how packed all over this code -- if you look carefully. Every Valve Source1 title ultimately goes through this layer on Linux/Mac. (The Mac ports do use a different, much earlier branch, however. At some point the Linux and Mac branches merged back together, but I don't know if that occurred in time for DoTA's version.)

We talked a lot about what we learned while working on this layer at GDC last year:

There's a lot of history to this particular code. This layer was first started by the Mac team, then later ported from Mac to Linux by the Steam team, and then finally ported by the Linux team to Windows (!) so we could actually debug it. (Because the best available GL debuggers at the time were Windows-only. We are working to correct that with vogl.) John McDonald, Rick Johnson, Mike Sartain, Pierre-Loup Griffais and I then got our hands on it (at various times) to push it down the correctness (for Source1) and performance axes. I spent many months agonizing over this layer's per-batch flush path: tweaking, profiling (with Rad's awesome Telemetry tool), optimizing, and testing it to run the Source1 engine correctly, quickly, and reliably on the drivers available for Linux.

The code is far from perfect: many parts are more like a battleground in there. It's optimized for results, and the primary metrics for success were perf vs. Windows and Source1 correctness, sometimes to the detriment of other factors. A lot of experiments were conducted, some blind alleys were backed out of, and we learned *a lot* about the true state of OpenGL drivers during the effort. If you want to see how to stay in the "fast lanes" of multiple Linux GL drivers simultaneously it might be worth checking out. (Most of the Linux drivers share common codebases with the Windows GL drivers, so a lot of what's in there is relevant to Windows GL too.)

(The first version of this post stated there was another version of togl that supported both Mac and Linux, and had all the SM3 fixes I made for various projects. Turns out the version on github is the very latest version, because all the togl branches were merged back into Dota2 at some point.)

Saturday, March 8, 2014

I've added Hitachi 6309 support to my disassembler, monitor interrupt handlers, and monitor client. So I'm now able to switch over my 6809 test app to use the 6309's "native" mode, which is something like 15-30% faster. I can single step over 6309 code sequences and display/modify the extra registers (E/F/V). I verified my disassembler by using the a09 6809/6309 cross assembler to assemble a bunch of test 6309 code, disassemble it, then diff the results vs. the original code. Lomont's 6309 Reference and The 6309 Book are the best references I've found.

The 6309 is amazingly powerful for its time. You've got some 32-bit ops, fast CPU memory transfers, hardware division/multiplication, various register to register ops, and two more 16-bit regs to play with over the 6809 (W and V, although V is limited to exchanges/transfers). W is hybrid register, useful as a pointer or general purpose register. I've written a good deal of real mode (16-bit segmented) 8086/80286 assembly back in the day, and I really like the feel of 6309 assembly.

Unfortunately, the assembler used by gcc6809 (as6809) doesn't support the 6309. The gcc6809 package comes with a 6309 assembler (as6309), but it doesn't compile out of the box. I got it to compile but it's very clear that whoever worked on it never finished it. I made a quick stab at fixing up as6309 but to be honest the C code in there is like assembly code (with unfathomable 2-3 letter variable names and obfuscated program flow), and I don't have time to get into it for a hobby project.

So for now, I'm using the a09 assembler (which does support 6309) to create position independent code (at address 0) contained in simple .bin files which I convert to as09 assembly source files. The .s files contain nothing but ".byte 0xXX" statements and the symbols. To get the symbols I manually place a small symbol table at the end of the .bin file that is automatically located and parsed using a custom command line tool which converts the a09 assembled .bin file to .s assembly files:

Monday, March 3, 2014

I've been working on a Linux OpenGL debugger for about a year now, so I figured it would be fun and educational to create a low-level CPU debugger just to learn more about the problem domain. (I'll eventually use all this stuff to remotely debug on various tiny microcontrollers, so there's some practical value in all this work too.) To make the effort more interesting (and achievable in my spare time), I'm doing it for the simple 6809/6309 CPU's and interfacing it to an old 8-bit computer (Tandy CoCo3) over a serial port. (Yes, I could emulate all this stuff, but there's not nearly as much fun in that. I want to work with *real* hardware!)

I first wrote a small monitor program for the 6809, so I could remotely control and debug program execution over the CoCo3's "bit banging" serial port. There's a bit of assembly to handle the stack manipulation, but it's written entirely in C otherwise using gcc6809. This monitor function lives in a single SWI (software interrupt) handler and only supports very basic ops: read/write memory, read/write main program's registers (which are popped/pushed on the main program's stack in the SWI handler), ping, "trampoline" (copy memory from source/destination and transfer control to the specified address), or return from the SWI interrupt handler and continue main program execution. The monitor also hooks FIRQ and enables the RS-232 port's CD (carrier detect) level sensitive interrupt so I can remotely trigger asynchronous breakpoints by toggling the DTR pin. (My DB9->CoCo serial cable is wired so DTR from the PC is hooked up to the CoCo's CD pin.)

With this design I can remotely do pretty much anything I want with the machine being debugged. Once the remote machine is running the monitor I can write a new program to memory and start it (even overwriting the currently executing program and monitor using the trampoline command), examine and modify memory/registers, implement new debugging features, etc. without having to modify (and then possibly debug) the 6809 monitor function itself.

The client app is written in C++ in the VOGL codebase and supports the usual monitor-type commands, plus a bunch of commands for debugging, 6809/6309 disassembly, loading DECB (Microsoft Disk Extended Color BASIC) .bin files into memory, dumping memory to .bin files, etc. It supports both assembly and simple source level debugging. You can single step by instructions or lines (step into, step over, or step out), get callstacks with symbols, and print parameters and local variables. I'm parsing the debug STAB information generated by gcc in the assembly .S files, and the NOICE debug information generated by aslink to get type and symbol address information.

Robust callstacks are surprisingly tough to get working. The S register is constantly manipulated by the compiler and there's no stack base register when optimizations are enabled. So it's hard to reliably determine the return addresses without some extra information to help the process along. To get callstacks I modified gcc6809 to optionally insert a handful of prolog/epilog instructions into each generated function (2 at the beginning and 1 at the end). The prolog sequence stores the current value of the S register into a separate 256-byte stack located at absolute address 0x100. (It stores a word, but the stack pointer is only decremented by a single byte because I only care about the lowest byte of the stack register. My stacks are <= 256 bytes.) The debugger reads this stack of "stack pointers" to figure out what the S register was at the beginning of each function. It can then determine where the return PC's are located in the real system hardware stack.

The 6809 code to do this uses no registers, just a single global pointer at absolute address 0xFE and indirect addressing:

Some pics of the monitor client app, showing source level disassembly, callstacks, symbols, etc. The monitor's serial protocol is mostly synchronous and I'm paranoid about checksumming everything (because bit banging at 115200 baud is not 100% robust on this hardware).

Here's the physical hardware running a heap test program. The cross platform C codebase compiles on both the PC using clang, and on the CoCo using gcc6809. I'm doing this cross platform because it's still *much* easier to debug on the PC using QtCreator vs. remotely debugging using my monitor app. Using the monitor to debug problems, even with symbols, makes me totally appreciate how good QtCreator's debugger actually is!

About Me

Back in the day I worked for several years at Digital Illusions on things like the first shipping deferred shaded game ("Shrek" - 2001), software renderers, and game AI. Then, after working for Microsoft at Ensemble Studios for 5 years as engine lead on Halo Wars, I took a year off to create "crunch", an advanced DXTc texture compression library. I then worked 5 years at Valve, where I contributed to Portal 2, Dota 2, CS:GO, and the Linux versions of Valve's Source1 games. I was one of the original developers on the Steam Linux team, where I worked with a (somewhat enigmatic) multi-billionare on proving that OpenGL could still hold its own vs. Direct3D. I also started the vogl (Valve's OpenGL debugger) project from scratch, which I worked on for over a year. In my spare time I work on various open source lossless and texture compression projects: crunch, LZHAM, miniz, jpeg-compressor, and picojpeg.